As global markets gyrate, a growing number of advisors are telling investors to set aside as much as 25% of their portfolio for shorter-term bets. Sizing up the risks and returns.

A fundamental tenet of financial advice is to settle on long-term goals, then pick an asset-allocation strategy and stick with it. A corollary is that trading in and out of positions in response to daily market gyrations is almost guaranteed to undermine an investor's long-term objectives.

But this advice, while often repeated, is not absolute. In the face of pervasive market volatility, some financial advisors recommend earmarking a portfolio for the short term and medium term, to take advantage of big market moves. This...